Electron discharge device



C. C. CUTLER ELECTRON DISCHARGE DEVICE Nov. 8, 1960 2 Sheets-Sheet 1Filed June 23, 1958 lNl ENTOR C.C?TL I? ATTORNEY QN/ V. E T B kblkbO 7 QN m H F y u a k E W, m. M 0 F x m A K 1 5 a i pm u\.u

2 Sheets-Sheet 2 C. C. CUTLER ELECTRON DISCHARGE DEVICE Nov. 8, 1960Filed June 23, 1958 United States v Patent ELECTRON DISCHARGE DEVICECassius C. Cutler, Gillette, N.J., assignor to Bell TelephoneLaboratories, Incorporated, New York, N.Y., a corporation of New YorkFiled June 23, 1958, Ser. No. 743,671

11 Claims. (Cl. 315-3) This invention relates to electron dischargedevices and, more particularly, to such devices which utilize anelectron beam to achieve signal amplification.

In my Patent 2,927,243, issued March 1, 1960, there is disclosed a noveltype of high gain, broad band amplifierwhich relies upon the exponentialgrowth of wave disturbances on a thin electron beam to produceamplification. The invention disclosed in that application involvesprojecting an annular or ribbon electron beam having a thickness whichis small compared to its circumference or width into a co-linearmagnetic field for flow along an extended path in the direction of themagnetic field. Signals to be amplified are applied to the beam in theform of circumferential or transverse modulations so that differentelectrons around the circumference or across the width of the beam areinfluenced to different extents by the applied signals. lation is in aplane transverse to the magnetic field and, as a result of this andbecause of the presence of space charge forces, the electrons develop atransverse displacement which, in turn, produces a sinusoidal modulationcircumferentially of or across the width of the beam. As the modulatedbeam travels along the longitudinal path defined by the magnetic field,an exponential growth of these sinusoidal disturbances on the beam takesplace. If the beam travels a sufficient distance the circumferential orwidth configuration of the beam tends to be broken into a plurality ofdiscrete clusters of spirally rotating electrons, a series of spiralnebulae. By proper positioning of an output transducer along the beampath an amplified replica of the input signal may be obtained at theoutput of the device. In such operation, the beam itself travels astraight path between input and output, and the amplification phenomenonis to be clearly distinguished from such devices which rely upondeflection of the beam from its path to produce amplification.

The significant advantages realized with such a device operated in theforegoing manner are that amplification is achieved over a wide band offrequencies from very low frequencies up to hundreds of megacycles, andthe amplification is achieved without benefit of interaction or wavepropagation circuitry as in klystrons and traveling and traveling wavetubes, other than the input and output transducers.

A disadvantage inherent in such operation, however, is the fact thatmaximum gain is achieved at low frequencies, the theoretical maximumbeing at direct current, and, although the bandwidth may be severalhundred megacycles it is not as great as it could be if the maximum gainoccurred at a frequency other than zero, assuming a symmetricalgain-bandwidth curve. vantage of such operation is that because maximumgain occurs at direct current, despite the large bandwidth, there isleft a significant gap of frequencies where no such order of gain andbandwidth is practically obtain- Such modu- Another disadable betweenthe maximum attainable with such opera- 2,959,706 Patented Nov. 8, 1960tion and the frequencies of operation of various other microwavedevices.

Accordingly, it is an object of the present invention to achieve in asingle device high gain over an exceedingly broad band of frequenciesfrom very low frequencies into the kilomegacycle frequency range.

It is another object of my invention to produce high gain, broadbandwidth amplification over any preselected band of frequencies fromzero frequency through several kilomegacycles.

It is still another objectof my invention to produce maximumamplification at anycdesired frequency within a broad band offrequencies from zero frequency through several kilomegacycles.

The present invention is based upon the discovery that a thin sheet beamor a thin annular beam can, under certain conditions, be made to flow ina magnetic field which is oriented diagonally with respect to thedirection of flow of the electrons in the beam. It has also been foundthat there is a direct relationship between the angle between themagnetic field and direction of flow and the frequency at which maximumgain is obtainable, such that for a given angular relationship, providedthe angle 'is small, the remaining parameters being fixed, the frequencyof maximum gain, or the midband frequency, is uniquely determined. Ithas further been found that, inasmuch as the gain of such a device isindependent of the angular relationship, a change in the angularrelationship does not alter the gain obtainable, and such an arrangementcan, therefore, be made to function as an amplifying band pass filter,the pass band of which can be shifted along the frequency scale by thesimple expedient of changing the angular relationship of the magneticfield and the direction of flow.

Accordingly, it is a feature of my invention that a thin sheet orannular electron beam,.which has been modulated in a transverse orcircumferential direction, is projected along a path of flow in amagnetic field and at an acute angle to the magnetic field.

It is another feature of my invention that the midband operatingfrequency of such an arrangement is varied by varying the angularrelationship between the electron beam and the magnetic field.

These and other features of my invention are achieved in a firstillustrative embodiment thereof which comprises an evacuated envelopehaving axially disposed therein at opposite ends an electron gun forforming and projecting a thin hollow beam and a collector electrode forcollecting the electrons in the beam. Located adjacent the electron gunis an input transducer means for modulating the electron beam withsignal energy to be amplified and located adjacent thecollectorelectrode is an output transducer means for converting modulations onthe beam into electromagnetic wave energy. Surrounding the envelope andextending longitudinally thereof parallel to the path of electron flowis an electromagnetic solenoid for establishing a magnetic field alongand parallel to the path of flow. Within the envelope and surroundingthe electron beam is a first shielding electrode. Along'the axis of thebeam and surrounded thereby is a second electrode comprising a metallicconducting rod or tube. Means are provided for producing a current flowin the rod such that there is produced in the region of the beam atransverse magnetic field. This field adds to the longitudinal field ofthe solenoid to produce a resultant helical magnetic field. Thus, amagnetic field is produced which is at an angle to the path of electronflow. By varying the current flow in the second electrode, the pitch ofthe helical field, and hence the angle thereof can be varied to producechanges in the midband operating frequency of the device. The shieldingelectrode and the rod are maintained at a potential difference withrespect to each other such that the beam does not deviate from its pathof flow under the influence of the magnetic field.

In another illustrative embodiment of my invention, an electron gun isutilized which imparts a rotational component to the beam, causing theelectors to travel helical paths. As a consequence, the direction offlow of the electrons is at an angle to the magnetic field. The angularrelationship between the field and the electrons can be varied byaltering the degree of rotation imparted to the beam.

' The present invention will be more readily understood from thefollowing description, taken in conjunction with the accompanyingdrawing, in which:

Fig. '1 is a sectional view of an amplifier tube embodying theprinciples of the present invention;

Fig. 2 is a graph which depicts the relationship between gain andfrequency for various angles between the magnetic field and the electronflow; and

Fig. 3 is a sectional view of another amplifier tube embodying theprinciples of the present invention.

' Turning now to Fig. 1 there is shown, by way of example, an amplifiertube 11 comprising an evacuated envelope 12 of glass or other suitablematerial enclosing .an electron gun 13 for forming and projecting anannular electron beam, and a collector electrode 14 for collecting thebeam at the downstream end of the tube. As used hereinafter, the terms.upstream and downstream will be used to designate points less remote ormore remote, respectively, from the electron gun with respect to otherpoints. Gun 13 includes a heater element 16, an annular cathode 17, anapertured beam forming electrode 18, and an accelerating anode 19apertured for passage of the beam therethrough.

In order that the annular beam may be modulated with signals to beamplified, an input transducer comprising grids 21 extending across theaperture in the beam forming electrode 18 are provided. This transducermay advantageously be of the form shown in Figs. 3A and 3B of myaforementioned patent. A source of signals, which, for simplicity, hasnot been shown, is connected between the cathode and grids 21. Such anarrangement produces a density modulation circumferentially of the beamwhich, in turn, produces a circumferential sinusoidal modulation in amanner clearly explained in the aforementioned patent. While the inputtransducer shown in Fig. 1 is of a specific form, it is to be understoodthat any one of a number of input transducers may be utllized tomodulate the beam in the desired manner. Examples of some of the varioustypes of input transducers which might be utilized are shown in myaforementioned patent. At the downstream end of the tube adjacent thecollector 14 is an output transducer 22 which functions in conjunctionwith collector 14 to produce an output signal. While the outputtransducer here shown takes the form of the output transducer shown inFig. 9 of my aforementioned patent, it is to be understood that it maytake any one of a number of forms, examples of which are disclosed inthat patent.

Surrounding the envelope and closely adjacent thereto is a solenoid 23for establishing a magnetic field which extends along the beam pathparallel to the direction of flow of the beam. Solenoid 23 functions ina manner to be explained more fully hereinafter to help produce amagnetic field along the beam path which is at an angle to the path offlow of the electrons in the beam, in accordance with the features ofthis invention. Within the envelope 12 and surrounding the beam path isa hollow cylindrical shield electrode 24 of conducting material.Extending along the axis of the tube and through the ends thereof is asecond electrode 26 which, as here shown, comprises a rod of conductingmaterial. As shown in Fig. 1, a variable source of potential 27 isconnected to the ends of the rod 26 for creating a current flow throughthe rod. As is well known, when current flows in a conductor atransverse magnetic field is created about the conductor. In the tube ofFig. l the current flowing in rod 26 in the interior of the tube createsa magnetic field which extends transversely to the path of flow. Thisfield thus created combines with the longitudinally extending magneticfield established by solenoid 23 to produce a magnetic field having aspiral or helical configuration in the region of the electron beam, asdesignated by the arrow B in Fig. -1. It can readily be seen that such amagnetic field a component which is at an angle to the direction of flowof the electrons in the beam. V

A voltage source 28 is utilized to apply a potential difference betweenshield electrode 24 and rod 26. The magnitude and direction of theelectric shield thus created by the electrodes 24 and 26 are such thatany tendency of the beam to be defocused or deflected because of theangular configuration of the magnetic field is overcome and the beammaintains its hollow cylindrical configuration throughout the path offlow. While certain important voltage sources have been shown in Fig. l,for simplicity various other voltage sources for properly biasingvarious of the other electrodes of the tube, as well as the signalsource for applying modulations to the beam, have been omitted.

In operation, signals to be amplified are applied to the grids 21 whichproduce a density modulation of the beam around the circumferencethereof, which in turn creates a circumferential sinusoidal modulationof the beam due to the resulting space charge fields created by thedensity modulation. The beam thus modulated travels along the length ofthe tube 11 toward the output transducer 22. As explained in theforegoing and as more fully explained in the aforementioned patent,these circumferential sinusoids on the beam grow in amplitude as thebeam travels along the length of the tube. Thus, at the outputtransducer the modulations on the beam represent an amplified version ofthe signal which was impressed on the grids 21. Output transducer 22 andcollector i4 convert these modulations on the beam into an output signalwhich is then abstracted for use.

In order that the significant features of my invention and theadvantages thereof may be more readily appreciated and understood, thefollowing analysis 'is presented.

This analysis is directed toward a thin sheet beam, but it is equallyapplicable to a hollow cylindrical beam of electrons inasmuch as ahollow cylindrical beam may be regarded as a special case of a thinsheet beam. It can be shown that the solution of the differentialequations of motion for electrons in a thin sheet beam under theinfluence of an electric field and a magnetic field parallel to the Zaxis is given by where C is a constant, t is time, and W is given by 1D2 l 52 Z [aweeen] and 'where a is one-half the separation between theinner and router electrodes (24. and 26 in Fig. 1). Equations 1 and T2are more general solutions for the dynamics of such :a system than havehitherto been available. They are readily verifiable by reference to anarticle entitled Instability of Hollow Beams by J. R. Pierce, I.R.E.Transactions on Electron Devices, vol. ED. 3, No. 4, October 1956, whereit can be seen that for small or Ilarge plate separations, Equation 2reduces to Equation ;27 or 52 respectively of that article.

Equation 1 describes the wave propagation in terms of displacementparallel to the X axis and the subscripts :refer to the roots ofEquation 2. In the frame of reference chosen in arriving at Equation 1,X and Z are coordinates which move with the average electron velocityand Z is parallel to the magnetic field. It is necessary, therefore, toconvert these coordinates to a stationary frame of reference. Taking theZ axis to be parallel to the direction of the beam, and using todesignate the angle between the Z axis and the magnetic field, the newcoordinates X and Z are given by X=X cos 0+Z sin 0Ut sin 0 (4) where Uis velocity in meters per second of the electrons in the beam and thesubscript 1 indicates the stationary coordinate system. In a like mannerthe propagation constants become =X sin 0Z cos 0Ut sin 0 which isapproximately which defines the frequency in terms of the angle 0 andthe parameters )8 and 'y.

From Equation 2 it can be seen that the maximum gain obtains when mmgain occurs at zero frequency. In the instant case, however, maximumgain still occurring at equal to 'zero in accordance with Equation 1, itcan be seen from Equation 10 that the frequency of maximum gain is abovezero, or more precisely, is given by the expression Qn fm1d-bund 27F a 0Also from Equation 1, it can be shown that Gain/unit length: -8.68 .j%(12 provided 0 is not larger. It will be noted that the expression forgain is independent of the angle 0 and is therefore the same for allpalues of 0 including zero, where the magnetic field is colinear withthe beam.

As pointed out in the foregoing, the preceding analysis was directed toa thin sheet beam, but is equally applicable to a hollow electron beamarrangement as shown in Fig. 1. In Fig. 2 there are plotted gain versusfrequency curves for a strip beam arrangement such as is shown in Fig.12 of my aforementioned patent for a number of values of 0, includingzero, and for the following values of the other parameters.

Beam current I =0.050 amp.

Beam voltage V =200 volts.

Beam thickness 2a=0.5 mm.

Beam width=1 cm.

Magnetic field B =876 gauss.

The voltage between the two electrodes on either side of the beam isvaried as the angle 0 is varied to achieve maximum focusing. It can beseen from the curves of Fig. 2 that while the maximum gain obtainable isnot affected by the angle of the magnetic field, the frequency band overwhich gain is obtainable is dependent upon the angle 0, as is thefrequency at which the maximum gain is obtained. It can further be seenthat for a given set of parameters and values of 0 equal to or greaterthan 0.1 radian, the band of frequencies over which gainis obtained isat least twice as great as the frequency band where the magnetic fieldis colinear with the electron beam. From the foregoing it is readilyapparent that by a proper choice of the angle 0, the desired operatingfrequency band and the desired midband frequency may be obtained withoutimpairing the gain obtainable. As Was pointed out in the foregoing, inthe arrangement of Fig. 1, the angle of the magnetic field relative tothe path of the electron can be adjusted by adjusting the amount ofcurrent flowing in the rod 26. Inasmuch as the angular field is theresultant of a longitudinal field and a transverse field, the angle mayalso be determined by varying the current in the solenoid 23. It canreadily be seen from Fig. 2, therefore, that the device of Fig. 1 can beoperated at any midband frequency within a large range by the expedientof varying the current in either rod 26 or solenoid 23, or both.

In the arrangement of .Fig. 1, the angle of the magnetic field relativeto the electron flow is varied to achieve the desired operatingcharacteristics by varying the orientation of the field itself. In Fig.3 there is shown a device wherein this angular relationship is achievedin a different manner. For simplicity, elements of the device of Fig. 3which are the same'as those of Fig. 1 are designated by the samereference numerals. The arrangement of Fig. 3 comprises an,,,amplifiertub .31 having an evacuated envelope 12 at one end of which is disposedan electron gun 13 and at theother end of which is a collector-electrode14. For reasons which will be apparent hereinafter, gun 13 is preferablyof the type shown and described in an article entitled AxiallySymmetrica Electron Beam and Magnetic Field Systems by L. A. Harris,Proceedings of the I.R.E., June 1952, pp. 700 through 708. Such a guncomprises an annular cathode 17 and a heater element 16. Adjacentcathode 17 is an apertured annular beam forming electrode 18 havinggrids 21 disposed in the aperture for modulating the beam with signalsto be amplified from a source of signals, not shown. Such modulation isof the type disclosed in my aforementioned patent and produces acircumferential modulation of the beam. Adjacent beam forming electrode18 is an accelerating anode 19 for accelerating the modulated beam. Forsimplicity, the various connections for supplying the proper operatingpotentials to the elements of the gun 13 have not been shown. The gun asthus far described is surrounded by a member 32 of magnetic material anda member 33 extending inside of cathode 17, electrode 18, and anode 19.Members 32 and 33 are connected together behind cathode 17 by atransverse magnetic member 34. Member 32 has an inwardly extendingflange 36 and member 33 has an outwardly extending flange 37. Togetherflanges 36 and 37 define an annular gap 38 which is aligned with theemissive surface of cathode 17, and the apertures in electrode 18 andanode 19. Surrounding member 33 is a solenoid 39 which is supplied withcurrent from a variable voltage source 41. Members 32, 33, and 34,flanges 36 and 37, and gap 38 define a magnetic circuit for the magneticfield generated by solenoid 39. It can be seen that within gap 38 thereis a magnetic field which is transverse to the path of electron flow asthe electrons enter the gap 38. As is eXplained in the aforementionedHarris article, as the electrons pass through the transverse field inthe gap, there is imparted to their motion a transverse component suchthat the electrons follow helical paths around the circumference of thebeam after emergence from the gap. Located within envelope 12 andsurrounding the path of the beam is an electrode 24 of conductingmaterial, and extending along the axis of the envelope is a secondelectrode 26. Electrodes 24 and 26 are maintained at a potentialdifference with respect to each other by a variable voltage source 42 sothat there is established between them a transverse electric field.Surrounding envelope 12 is a solenoid 23 which establishes alongitudinally extending magnetic field along the path of flow. By aproper choice of the voltages on electrodes 24 and 26, the electrons inthe beam can be made to follow helical paths along the length of thetube such that they travel at an angle with respect to the magneticfield established by solenoid 23. At the downstream end of the tube isan output transducer 22 which functions with collector 14 to produce anoutput signal. Transducer 22 can take any one of a number of formsdisclosed in my aforementioned patent,

In operation, the angle between the electron paths and the magneticfield can be adjusted by varying the current in solenoid 39 so that anydesired angle, and hence any desired midband operating frequency may berealized so long as the angle is small.

The devices of Figs. 1 and 3 both utilize annular beams. It is to beunderstood, however, that the principles of the present invention areequally applicable to strip beam amplifiers utilizing thin flat sheetbeams such as the type disclosed in my aforementioned patent.

The specific embodiments herein disclosed are intended to beillustrative of the principles of the present invention. Various otherarrangements may be devised by one skilled in the art without departingfrom the spirit and scope of the invention as set forth in the-appendedclaims.

ducing a plurality of distinct transverse bunches of electrons in thebeam, said means including input meansupstream along the path of flowfor modulating the beam periodically along a transverse dimension with asignal to be amplified, output means downstream along the path of flowresponsive to the modulations on said beam for producing an outputsignal, said input and output means being separated by a drift region,and means for maintaining the transverse dimensions of said beamsubstantially constant within said drift region between said input andsaid output comprising means for establishing in said drift region amagnetic field, said field being at an angle 0 with the electron pathsin said beam, such that the midband operating frequency of said deviceis given by the expression 71 f- 27F tan 0 where U is velocity in metersper second of the electrons in the beam, and '7 is the propagationconstant transverse to the axis of the beam of the signal disturbance onthe beam, and tan 0 is approximately equal to 0.

2. An electron discharge device for amplifying signals over a broad bandof frequencies comprising, in combination, means for forming andprojecting a hollow cylindrical electron beam along the path, thethickness of said beam being small compared to its diameter, means forproducing a plurality of distinct transverse bunches of electrons in thebeam, said means including input means upstream along the path of flowfor modulating the beam periodically around the circumference thereofwith a signal to be amplified, output means downstream along the path offlow responsive to the modulations on said beam for producing anamplified output signal, said input and output means being separated bya drift region, and means for maintaining the transverse dimensions ofsaid beam substantially constant within said drift region between saidinput and said output comprising means for establishing in said driftregion a magnetic field, said field being at an angle 0 with the path ofelectrons in said beam, such that the midband operating frequency ofsaid device is given by the expression f=g tan 0 7r where U is velocityin meters per second of the electrons in the beam, and 7 is thepropagation constant transverse to the axis of the beam of the signaldisturbance on the beam.

3. An electron discharge device as claimed in claim 2 wherein the meansfor establishing a magnetic field comprises a first means establishing alongitudinal magnetic field along the path of flow and a second meansfor establishing a transverse magnetic field along the pa of flow.

4. An electron discharge device as claimed in claim 2 wherein the meansfor forming and projecting the beam comprises means for imparting to theelectrons -a transverse component of velocity such that the electrons inthe beam follow helical paths around the circumference of the beam, andthe means for establishing a magnetic field comprises means forestablishing a longitudinal magnetic field along the path of flow.

5. An electron discharge device for amplifying signals over a broad bandof frequencies comprising, in combination, means for forming andprojecting a hollow cylindrical electron beam along a path, thethickness of said beam being small compared to itsdiameter, means forproducing a plurality of distinct transverse bunches of electrons in thebeam, said means including input means upstream along the path of flowfor modulating the beam periodically about the circumference thereofwith a signal to be amplified, output means downstream along the path offlow responsive to the modulations on said beam for producing anamplified output signal, means for establishing a drift region betweensaid input means and said output means, and means for maintaining thetransverse dimensions of said beam substantially constant within saiddrift region between said input and said output comprising means forestablishing in said drift region a magnetic field, said last-mentionedmeans comprising magnetic means surrounding the envelope andestablishing in said drift region a longitudinal magnetic field andconducting means extending along the axis of said envelope, means forproducing a flow of current in said conducting means whereby atransverse magnetic field is established in said drift region, themagnitude of said longitudinal field and said transverse field beingsuch that the resultant magnetic field is at an angle with the electronpaths in said beam such that the midband operating frequency of saiddevice is given by the expression where U is velocity in meters persecond of the electrons in the beam, and 'y is the propagation constanttransverse to the axis of the beam of the signal disturbance on thebeam.

6. An electron discharge device as claimed in claim wherein said meansdefining a drift region comprises a hollow cylindrical member ofconducting material surrounding said beam path, in further combinationwith means for maintaining said hollow cylindrical member at a potentialdiiference with respect to said conducting member. 1

7. An electron discharge device for amplifying signals over a broad bandof frequencies comprising, in combination, means for forming andprojecting a hollow cylindrical electron beam along the path, thethickness of said beam being small compared to its diameter, said meansincluding means for imparting to the electrons in the beam a transversecomponent of velocity whereby they flow in helical paths around the beamcircumference, means for producing a plurality of distinct transversebunches of electrons in the beam, said means including input meansupstream along the path of flow for modulating the beam periodicallyabout the circumference thereof with a signal to be amplified, outputmeans downstream along the path of flow responsive to the modulations onsaid beam for producing an amplified output signal, means forestablishing a drift region between said input means and said outputmeans, and means for maintaining the transverse dimensions of said beamsubstantially constant within said drift region between said input andsaid output comprising means for establishing in said drift region alongitudinal magnetic field, such that the magnetic field is at an angle0 with the electron paths in said beam such that the midband operatingfrequency of said device is given by the expression where U is velocityin meters per second of the electrons in the beam, and 'y is thepropagation constant transverse to the axis of the beam of the signaldisturbance on the beam.

8. An electron discharge device as claimed in claim 7 wherein the meansestablishing a drift region comprises a hollow, cylindrical member ofconducting material surrounding said beam path and a longitudinallyextending member of conducting material disposed along the beam pathWithin the annular beam, and means for maintaining said outercylindrical member and said longitudinal extending member at a potentialdifierence with respect to each other.

9. An electron discharge device for amplifying signals over a broad bandof frequencies comprising, in combination, means for forming andprojecting a thin electron beam, means for producing a plurality ofdistinct transverse bunches of electrons in the beam, said meansincluding means for causing wave disturbances transverse to said thinbeam in accordance with input signals, means defining a drift region,means for maintaining the transverse dimensions of said beamsubstantially constant within said drift region, said means includingmeans for generating a linear magnetic field in said drift region forexponential growth of said wave disturbances, said linear magnetic fieldbeing at a small angle to the direction of the electrons in said beamand said angle being within the range wherein the angle is substantiallyequal to the tangent of the angle.

10. An electron discharge device in accordance with claim 9 wherein saidmeans for generating said magnetic field includes a first means forgenerating a linear magnetic field parallel to the axis of said beam andsecond means for generating a magnetic field transverse to said beamaxis.

11. An electron discharge device in accordance with claim 9 wherein saidmeans for forming and projecting said beam includes means for impartingto the electrons in said beam a transverse component of velocity wherebythey flow in helical paths in said drift region.

References (Iited in the file of this patent UNITED STATES PATENTS2,424,965 Brillouin Aug. 5, 1947 2,610,308 Touraton et al. Sept. 9, 19522,654,047 Clavier Sept. 29, 1953 2,761,088 Warnecke et al. Aug. 28, 19562,811,663 Brewer et al. Oct. 29, 1957 2,830,223 Mihran Apr. 8, 1958

